PWM simple questions

Discussion in 'General Electronics Chat' started by keithwins, Jan 28, 2014.

  1. keithwins

    Thread Starter New Member

    Jan 28, 2014
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    I hope you can help with some simple PWM questions.

    I've got an old treadmill I'm trying to reclaim. It has a PWM motor (burned out controller) that is marked as such:
    120VDC
    18A
    2.5 HP
    3800 RPM
    1865W

    As a first pass, I want to just buy a (cheap) PWM controller and play with it, but I can't find many that get up to 120VDC: for example, here are stats on one:
    Specification:
    • working voltage: DC10v- DC 50V
    • Rated current: 40A(Maximum output current)
    • Control Motor Power : 0.01-2000W,
    • working voltage 50V: 50V*40A=2000W(max)
    • quiescent current:0.025A
    • PWM Regulation range:5-100%
    • PWN Frequency : 12khz
    So here are my ill-informed questions:
    If I drive my motor with lower voltage (than it's rating), do I risk burning it out, or have inadequate power from the motor? (those are my two fears).
    Does the controller listed here seem capable of powering this motor?

    Can you help clarify any of this for me? Sorry they are such basic questions.

    K
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    No you won't burn it out, just that it will run proportionally slower.
    If you want a fairly cheap controller but not PWM, there are always KB/Baldor SCR drives on ebay for as low as $20.00.
    The controllers shown also usually require a power supply of some kind.
    10v-50v.
    Max.
     
  3. keithwins

    Thread Starter New Member

    Jan 28, 2014
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    Thanks Max! I guess I am still a little confused: speed is first and foremost proportional to pulse duration, no? So how exactly does V come in there? And I guess my concern is that if there isn't enough V to meet a power demand, I'd get a locked rotor condition... but either you're saying that won't happen, or it doesn't matter if it does (won't burn out the motor)?

    I guess my assumption would be that power would be primarily proportional to V, and speed to pulse duration/width... And A will take care of itself, as long as my power supply can meet the demand. Or am I still wrong? Sorry if I'm being uncooperative! Just trying to be sure I understand. Thanks again.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    No I just meant that running a DC motor on lower voltage does not in and of itself cause problems, just that the rpm is directly proportional to the applied DC level.
    Power is proportional to current.
    If you have no current limit protection or limit level on the drive or controller you are using, then it is possible to damage the motor, regardless of voltage if presenting too large a load.
    If using a PWM controller then the voltage is proportional to the pulse width and mean current is dependent on load.
    On a DC motor, Continuous Torque rating (current) is usually rated at maximum at zero rpm, reducing as rpm increases up to the rated rpm, most high end DC motors this curve is fairly flat.
    Peak Torque is a rating above continuous torque and cannot be exceeded without damaging the motor, and should only be experienced for a very short duration, such as accelerating/decelerating a load.
    Max.
     
  5. keithwins

    Thread Starter New Member

    Jan 28, 2014
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    Thanks again Max! I got all that except this:

    If using a PWM controller then the voltage is proportional to the pulse width.

    Maybe if I thought about it more this would make sense, but it isn't what I expected. I sort of thought of pulse width and voltage as separate in PWM.

    Is there some sense of averaged voltage, vs. (constant) instantaneous voltage of each pulse? Sorry I'm sure I'm saying this sloppily.

    K
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I could have also stated that the maximum possible applied DC is going to be the applied DC power source level and 100% pulse width.
    e.g. if the applied DC power is 50% of the motor rated voltage and the pulse width were 100%, this would result in 50% of the motor rated rpm, considering a no load condition.
    Max.
     
  7. keithwins

    Thread Starter New Member

    Jan 28, 2014
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    I'm not finding these SCR drives, I guess I don't know what I'm looking for?

    K
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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